Flow/Steric Field-Flow Fractionation (flow/StFFF) is a recently developed method with extraordinary speed and resolving power designed to separate particulate materials falling roughly in the 1-100 um particle diameter range. The object of this proposal is to examine the potential of converting this research finding into a fast and reliable tool for the fractionation and characterization of cells in samples of clinical and biochemical interest. In flow/StFFF sample particles are subjected to a crossflow field while being carried along by a carrier liquid in a ribbon-like thin channel with permeable walls. Cell particles will elute depending on their size and shape, and in some cases, density. As these parameters are reflected in the elution profiles or fractograms, one should be able to identify the difference between normal and abnormal cells by observing the pattern of the fractograms. This method will be studied for its possible routine application to cell populations. In particular, we will examine its applicability to the diagnosis of abnormal human red blood cells. The ability of flow/StFFF to isolate pure cell populations of viable cells will also receive major attention. The proposed method, once developed, should be relatively simple and cost effective. The analysis time with this method is now approximately 1-5 minutes for a sample, with additional reduction possible. From 10.5 to 10.7 cells are expected t constitute the optimum sample size.